Generally, there are two types of input/output methods from a host computer to a storage device, input/output (I/O) per block (block I/O) and I/O per file (file I/O). The storage devices which can accept the latter type are called file storages, network attached storage (NAS) or others. Recently, storage devices including both an interface for block I/O and an interface for file I/O have spread, and specifically, technology capable of processing both block I/O requests and file I/O requests for a single logical volume is suggested (refer to the Patent Document 1 for an example).